3.8kWh Solar setup in LA - Do I really need a powerwall?

3.8kWh Solar setup in LA - Do I really need a powerwall?

Just wondering your thoughts on if it's worth it to get a powerwall along with a small solar setup.
I'm questioning if the powerwall would even be able to charge daily for use during peak hours.
Would I be better off just sending the excess back t the grid?
Anyone have some thoughts on this?

Atom12 | 8 December 2019

If you are expecting service disruptions, then yes. Get the PW. Here in the Northern California fire zones (PG&E), I expect outages to increase dramatically. Also, my PW's included with my solar installation get the Federal tax incentive.

gregbrew | 8 December 2019

It's rarely "worth it" to get a Powerwall (PW). Solar...yes., unless you value a PW as "insurance" against outages, which is especially important if you have medical devices in the household, like a C-PAP. (Fortunately we're not there, yet.) It's expensive "insurance", but it's why I got two PWs here in "Earthquake Country", Southern California. Even without earthquakes, my PWs have protected my refrigerator and freezer food, as well as my wine collection, during a planned outage of 10 hours, and several shorter, unplanned outages in the 10 months since installation. I've watched the e-grid become less and less reliable during the 50+ years I've lived here. You simply cannot count on it to be here any more.

In the States, it's almost always against utility rules to routinely charge a PW from the grid. You'd want to do this in order to "time-shift", filling the PW with energy from low rate times to export it during high rate times in order reap cost differential between the two, but it's not enabled in the PW firmware here. It *is* enabled in other locations, like Australia.

bcmusik | 8 December 2019

Thank you both for the valid answers. Just what I needed.
Gregbrew, since you're also in SoCal, do you think a small system like this would really be able to re-charge a single Powerwall daily? (solar only of course - not from the grid). I don't get many power outages here in Burbank so the PW would only be to provide energy during Peak high cost hours and maybe into the night.

Passion2Fly | 8 December 2019

I agree with @gregbrew. Hard to justify it financially...
I have one PW and a small 5kW solar. I save about $400-$500 per year with load shifting in San Diego... I have spreadsheets to track my power consumption and I calculate the savings based on the PW data...

gregbrew | 9 December 2019

I have 5kW of PV and two PWs. It took two days to charge the PWs after installation in late January/ Early February this year, which is during my low production months. My PV array is situated almost ideally, on a South-facing pitched roof. Having a large South-facing roof was one of the top criteria for picking this house.

One PW will hold 13.5kWh, which a 3.8kW array can probably fill in Burbank during most of the year. Depends on the array orientation, though. In the dead of solar Winter (pretty close to right now), my 5kW will still produce about 20kWh if it's clear. It can be close to zero in the rain. During Summer, it routinely produces 30kW. You can extrapolate from that for 3.8kW, but understand that I'm subject to a pretty mean marine layer most mornings until 10AM or so. You probably don't get much of that in Burbank, which should help.

You really need to look at your e-bills to get a feel for your routine daily usage (by the hour), if you intend to use a PW to time-shift grid usage to the high power rate evening hours (TOU). Also understand that due to Physics (2nd Law of Thermodynamics) the energy transfer into and out of a PW is not free. You're converting electrical energy into a chemical form (and vice-versa) in the batteries. There's about a 10% energy loss in this in-out energy conversion process. Take that into consideration in your calculations, if it's close.

gregbrew | 9 December 2019

I wish this forum had an editing feature: The "30kW" in the second paragraph should read "30kWh". The batteries hold *energy* (kWh), not power (kW).

charlesj | 9 December 2019

@bcmusic, you should be able to see your solar productions during sunlight times how much is being produced and how much returned to the grid. If your powerwall is down, that will be used to recharge. How quickly depends on this and who you set up the PW, backup only or to supplement the usage as needed.

No, there is no cost benefit to this other than a piece of mind when grid is down. An auto backup system may be cheaper but it too depends on fuel of some kind.

bcmusik | 9 December 2019

You're replies are much appreciated you guys. Thanks for taking the time.
I feel like I can easily get by without it. I think there's just a part of me that wants one (like when I got my model 3).
I'm leaning towards canceling the powerwall part of my purchase (assuming I still can). Thanks again for the info/advise.

calleserra | 15 December 2019

I just went through this exercise of justifying the powerwall purchase after I placed the order today. Looked at discussions on reddit and various forums. I just realized that with net metering 2.0, getting a PW isn't worth it. It's because the excess kwh generated by the solar panels can be exported and "stored" in the grid (instead of storing in the PW which costs $11,000 less fed credit and waitlisted SGIP) and used in the form of generated bill credits. Sort of like storing your rollover minutes (excess kwh generated) in the cloud (the grid). I cancelled the PW order and will just get the solar panels.

PapaSmurf | 15 December 2019

The Powerwall is bada$$ cool.

Forget the economics of it. It is just freakin' awesome, pure and simple as that.

I get an incredible amount of satisfaction just turning off my connection to the grid and going pure off grid mode. As soon as my system was installed, but the net meter was not yet installed, I was off the grid for a few weeks. I didn't want my system just sitting around being unused.

The day the county inspector approved my system, I went off grid. For me, I love the feeling of not needing Duke Energy, which is truly an evil company and their customer service department deserves every bad thing that happens to their senior VPs.

mr.ramonlao | 16 December 2019

Considering 17.6 KW Solar system with 2 PW2. Just here to hopefully learn more about benefits of PW2 before contract obligation.

Passion2Fly | 16 December 2019

Did the installer approve your project? 17.5 kW is too much for only two Powerwalls. Technically, the output of your solar needs to be less than the total capacity of the batteries. Each battery can sink 5kW, so your system would require at least four PowerWalls...

Passion2Fly | 16 December 2019

I think Tesla requires one PowerWall for each 7.5kW of solar for this very reason... They do allow some overhead due to the fact that full output is never really achieved and your home will be sinking some power too... So, your system would be ok with probably three Powerwalls minimum...

gregbrew | 16 December 2019

I'm having trouble finding any documentation from Tesla stating that there's a limit on how much PV can accompany Powerwalls. I believe it boils down to how much backup capacity you want, and how long you want to wait to refill your batteries, up to the power I/O limit of each PW, which is 7.2kW peak, 5kW continuous. If the PV output is higher than the PWs can take, the TEG should be able to deal with it, either by shuttling the excess to the grid, or cycling the PV as in an outage with full batteries and sunlight.

Atom12 | 16 December 2019

@Passion2Fly: "I think Tesla requires one PowerWall for each 7.5kW of solar for this very reason."

Where did you see this? I'd never known that and knowing what I currently know sounds arbitrary or purely regulatory. What about PW installations with no solar?

Passion2Fly | 16 December 2019
Passion2Fly | 16 December 2019

thegateway can only control the solar inverter ON/OFF. At least for the older ones like mine...
So, if the solar output is higher than what the PW can sink (which varies with the battery's internal temperature), the gateway will just shut the solar off... probably, towards the end of the day when the production drops, the gateway will turn your solar back on... unfortunately, you missed a good amount of solar energy production...

gregbrew | 16 December 2019

That link looks like an installation performance recommendation rather than an actual requirement or hardware limitation. I've seen nothing that indicates that the TEG can only manage up to 7.5kW of PV per Powerwall.

smaches | 16 December 2019

Interesting discussion. My understanding from a schematic viewpoint: House = load... batteries are wired as source for this load, and the batteries are connected to the inverter and the grid to 'balance' the load through the TEG. The controller in the TEG will 'sense' all these variables and determine best practice for the current (no pun intended) configuration. When the batteries are full, the power still 'floats' through the PWs to maintain the house load. Which is why you can see the app show a draw from the array and the batteries during the sundown transition. Cool Beans, eh?
Which is why they (TESLA) originally spent quite a lot of engineering resources to determine the 'best' setup for each unique user if installing a paired system (contract signed in October 2017 and my PTO was July 2018 - whew).
They're shifting away from that design concept with a menu of standard sized systems now, but that's another story.

jrweiss98020 | 16 December 2019

bcmusik: Part of the answer to your initial question is another question: What can you afford?

If you will have to finance the Powerwall, and you don't really need it, then don't buy it. If you are financially stable, and can afford to spend some $$ on a backup system, it may be worth it.

Patrick | 16 December 2019

Don’t have a schematic handy but perhaps this photo of our TEG wiring will help explain all the connections.

The “microgrid” panel (installed left of TEG - not shown) combines the Powerwalls and inverter with separate breakers for each. The microgrid panel connects to the TEG on the large bottom lugs, which also connect to the main breaker panel of the home. The utility grid connects to the upper lugs on the TEG.

So everything (grid, PWs, inverters, home) is physically connected together when the grid is up, with the TEG controller monitoring everything via CTs and the Neurio box and orchestrating the energy flow as required.

When the grid fails, the TEG just disconnects the grid and everything else just keeps on humming. So simple, so elegant.

Haven’t done much research on restrictions for matching the various capacity of the PV, inverter and PWs but it stands to reason that one should install a PV generator with somewhat higher power output than the max expected consumption load of the home during the day (so the PWs can also charge) but not much higher. Anything more is wasted unless one has net-metering and the local utility pays good rates to buy the excess generation.

Tesla or a good solar dealer/integrator will usually guide the customer to make the right selections in this regard.

Passion2Fly | 16 December 2019


As much as I like the Tesla PowerWall, I wouldn't qualify their TEG as the "invention of the century"... It actually acts as an ATS (Automatic Transfer Switch) which has a been around for a while with backup generators... The battery management software (inside the battery) is really the brains of the operation... The TEG is an ATS + communication box via the Neurio (between the battery and the outside world)... The Neurio also senses the currents via the CTs...

Something else you need to understand is that the TEG cannot control the direction of every single electron. So, the PW is not ONLY "charging" from the solar. It is charging from Grid + Solar but the CTs make sure that the charge going into the PW is exactly equal to the output of your solar system, so they can "claim" that the PW is charging from "solar"...

The principle is simple: Kirchhoff’s Current Law
“The algebraic sum of all currents entering and exiting a node must equal zero”

Patrick | 16 December 2019

Sounds like we’re splitting hairs here and we both understand how the system works. Enjoy your microgrid!

infofiles | 18 December 2019

I was wondering how much do you actually generate annually with small and medium system in southern California. My average daily usage bases on last 12 months is 30 kwh. Powerwall is looking bleak on ROO give how expensive installation costs are. I was thinking to subscribe small for $65/month and wondering how much my bill be offset? annually I can only get 6000kwh, cost per kwh comes to $0.13/kwh plus .for solar subscription. On TOU Prime, except 4-9 pm rate from SCE is 0.13/kwh any way.

What has been experience of others who have usage about 30kwh/day and using small solar system? Are you seeing any savings?

Passion2Fly | 19 December 2019

In your case, leasing is too expensive. The same system (4 kW) installed with tax incentives will produce ~$0.07/kWh over 20 years... but you need to pay upfront...

infofiles | 19 December 2019


what about if 7.6KW system is used?

gregbrew | 19 December 2019

I'm in So. CA. Huntington Beach, specifically. We *are* subject to a marine layer most mornings until 9AM or so. We have a 5.035kW DC array (19 each of 265W panels) with a 4.2kW AC ABB inverter and two Powerwalls. The peak production I've ever seen is during April 2017, and it was 4.6kW instantaneous, where it clipped. That output from the array is very rare. Our pitched roof is directly South-facing...a near ideal installation. We added the PWs in January of 2019 to the PV that was installed in December 2015. Our contract stipulates 8.4MWh per year of production. On average, we've hit that to within 1%. Amazing that the Solar City installation algorithms were so close to reality.

I look at the Powerwalls as a net cost for "insurance", as there isn't any benefit to time-shifting for our usage patterns. I've yet to see a cost/benefit analysis that shows *any* ROI for Powerwalls in SCE country. The fixed daily costs with SCE TOU wipes out any benefit. We've just stayed with residential tiered, as we're grandfathered in, and it makes the most economic sense for us. New PV/PW installations are forced into one of a couple of TOU options. The "good" (for solar customers) TOU options were eliminated a year or two ago. SCE has finally figured the best tariff model for their *stockholders*. Pay attention to fixed costs, and use the *entire* bill to calculate per kWh costs. There's more to the bill than just the SCE published tariff tables, and those are not easy to find.

Passion2Fly | 19 December 2019

That’s even worse since the average price per kWh will be $0.14. It’s more than what SCE is charging you...
The only problem with SCE is the peak price of $0.38/kWh. You need to offset this with either a PowerWall or by minimizing your power usage during this time (4PM-9PM).

The only way to win is by purchasing the system. The leasing is too expensive, given the SCE’s unbelievable rates... Just FYI, my rates in San Diego are $0.28/kWh during the day in winter and $0.09 from 12AM to 6AM for EV charging...

Passion2Fly | 19 December 2019

Did you look into a solar loan with your bank?

Passion2Fly | 19 December 2019

I agree with @gregbrew. You need to calculate your effective rates by adding the fixed costs imposed by SCE. However, I would still not recommend leasing in your case... the return on investment seems negligible...

PapaSmurf | 22 December 2019

There is no requirement of number of powerwalls to solar kW size,

My system is 16.38 kW and we only installed two Powerwall 2 units with it, installed in November 2019 by Tesla.

When the system is off the grid, the maximum rate of recharge is about 10.0 kW to the two Powerwalls. If it exceeds that for more than a few seconds, the inverters go into pause mode for 300 seconds and the house flips to running in battery mode. Then after 300 seconds, the solar comes back online to see if it can run again. If not, another 300 second pause. We have two Solar Edge inverters, one with 24 panels, the other with 28. Sometimes it will only operate one inverter and the other will be in 300 second pause mode. It does this to keep the recharge rate below 10.0 kW.

When I observed it happening, solar panels producing around 11 to 12 kW, I increased the house energy consumption by starting a load of clothes washing and a load drying, so that the rate of PW recharge was below 10.0 kW. Then the solar production can run at full power.

When connected to the grid, it doesn’t matter. The TEG regulates flow of energy so that maximum recharge rate to the PW is not exceeded. The excess energy goes to the grid.

Looking back now, I wish I had purchased a third Powerwall.
16.38 kW can recharge two Powerwalls very quickly even on a partly cloudy day in December.
My system can recharge two Powerwalls from 5% to 100% before noon, even when supplying the morning house electrical load of dishes, washing clothes, electric water heater, etc.
I will likely add a third Powerwall in the future at some point.

Passion2Fly | 22 December 2019

How did you manage to monitor TWO solar inverters with a single gateway? Don’t you need TWO sets of solar CTs? The gateway only has one set, no? Thanks!

Passion2Fly | 22 December 2019

Unless the two solar inverters are interconnected at a dedicated sub-panel....

Patrick | 22 December 2019

@Papa - thanks for sharing the data points. Just starting to get familiar with our new system but it seems like a good general rule of thumb for deploying Powerwall microgrids is using a minimum of one PW for each 5 kW of solar power generation.

We went with a higher ratio (3 PW on a 10 kW generator) to increase the odds of staying at or near 100% self powered during longer periods of expected lower generation in the late fall and winter months. We’ll see what happens.

In your case, what is your average daily generation? Average home consumption? How much do your two PWs discharge every night before recharging the next day?

Patrick | 22 December 2019

@Passion - you nailed it in your second post. We have two inverters on a second home (no PWs) and they are paralleled in the generation subpanel. Same could be done in a full microgrid generation panel feeding a single TEG.

Patrick | 22 December 2019

The CTs get connected to the combined output of the generation panel.

PapaSmurf | 22 December 2019


There is a small readout screen and button on my SolarEdge inverters. If I press the button for "OK" then it gives me a readout of the instantaneous output for the solar panels associated with that inverter. I can see AC voltage, DC voltage, total number of watts being produced dynamically changing every split second, etc.

Inverter #1 controls 24 panels, primarily the east facing morning panels.
The other inverter #2 controls 28 panels, primarily south and west facing.

It is quite easy for me to see the difference early morning when inverter #1 is producing the early morning large amounts of solar energy. Then over the course of the day Inverter #2 catches up and then provides the bulk of the solar energy in the afternoon.

This data is not visible in the app or the TEG. It is visible on the screen of the SolarEdge inverter in my garage.

So when I see on the app that my batteries are approaching 94% to 95% while Off-Grid, I can walk into my garage and visibly see the inverters go into a 300 second pause because there is now a 300 second countdown on the inverter screen. Then at the end of that 300 second period, they cycle back online and test the system to determine whether the battery has declined enough for more solar recharge energy to be accepted. Or it is checking to see if the solar input exceeds 10.0 kW or now. Sometimes it decides to only turn on one inverter so either 24 or 28 panels are sending energy to the Powerwalls. It is sort of random which inverter is online and which goes into a 300 second pause cycle.

PapaSmurf | 22 December 2019

We have had our system since late November, so this is the worst time of the year.

16.38 kW nameplate maximum. 52 panels at 315 watts each.
Two SolarEdge inverters, two Powerwall 2 units.

The maximum I saw our system produce during December was approx 12.7 kW during the time frame of 11:45 am to noon. I regularly saw our system exceed 11 kW to 12 kW even during partly cloudy days. Our system would spend several hours above 9 kW (10 am to 2 pm) each day during December. And it would easily be above 7 kW by 9 am and above 7 kW till 3 pm. And this is likely during the worst month of the year during December. I think having panels aimed East and West is working out well for us. To fit 52 panels on our roof they used just about every surface available except the North facing roof angles.

So for December 2019, we regularly exceeded 60 kWh per day, and a few days 70 kWh, in solar production. Based on my brother's system that he has had for two years, I expect to easily exceed 110 kWh per day during the summer (better sun angle and longer days) on a cloudless day.

Consumption. We have a purely electric house. There is no natural gas in my neighborhood. Electric cooking, electric oven, electric water heater, electric home heating, electric dryer. It is a two AC unit house, 3500 sq ft. So both AC units being on will consume about 3,000 watts (3.0 kW) and stay that way for hours at a time on a hot day. All day and all night. Anyone hopping into the shower (4 kids, me and wife) and the hot water heater comes on at about 4,500 watts (4.5 kW) for 10 to 20 minutes. If someone starts cooking in the kitchen, that is easily 1,500 to 2,000 watts during that time frame.

God forbid that we have to heat the house with electric. I found out that can spike energy consumption to 20,000 watts for upstairs and downstairs at the same time. That's right, 20 kW for electric heat upstairs and downstairs on a cold day/night when the temperature can drop to 30-40 degrees in Florida. When I was off grid and tested that on the two Powerwalls, that tripped the entire system into shutdown mode. If solar output were at 11-12 kW and both Powerwalls were over 90% providing 10 kW of continuous output, I figure we could run the electric heat in the house for maybe 2 hours in the middle of the day. Then the batteries would be near empty and we would need to scale back consumption to bare minimum. And then the batteries would be too empty to get thru the night.

If I manage things at night while off grid, two Powerwalls are enough to get through the night with most of the first world intact. Just by turning off the electric water heater and the AC units on the power panel, that is enough to remove the heaviest power consumers. Then I will usually have plenty of energy in the morning still (30% to 40%). There is plenty to turn on the electric water heater at 6 am for morning showers. Then the sun rises and there is plenty of energy to run just about everything in the house, even while off grid.

I have intentionally taken the battery packs down to 5% and they recharge quickly to 96% by noon. When off grid the batteries don't seem to want to go above 96%. They will only go to 100% when connected to the grid so they can dump excess energy to the grid while doing a slow charge the final stretch to 100%.

With a system the size of ours at 16.38 kW, I would definitely get more than two Powerwalls. It works fine with two Powerwalls, but I find the 10.0 kW continuous limits to be restrictive. Recharge rate and discharge rate is limited to 10.0 kW. So when the system is producing 12+ kW and our house is only consuming less than 1 kW, the system (while off grid) will shutdown because it cannot handle so much energy recharging the Powerwalls.

I actually just today placed an order for two additional that I will in a few months have four total. That way I will have 20 kW max input or output. That should provide plenty of extra margin to get through the night and the next day following a hurricane outage here in Florida.

Also, having four Powerwalls will allow me to recharge a Tesla at 240 volts 80 amps (19.2 kW) while being off the grid.

First World Problems !!! Yay !!!

PapaSmurf | 22 December 2019

If your house is like mine, the Tesla solar/pw app indicates that the house is always consuming at least 0.4 to 0.5 kW, even at night with everyone asleep and most lights off. Just running the kitchen refrigerator, garage deep freezer and night lights seems to consume a constant 400-500 watts all night long.

So if we count evening/night hours as 5:30 pm sunset till 7 am sunrise (my December approx times), that is 13 hours and 30 minutes at 400 watts, before we even do anything else at home. That is approximately 5 to 7 kWh of nightly energy consumption just to power the bare minimum basics overnight. With a one PW2 system, that is about 50% of your storage. For a two PW2 system, that is about 25% of your storage.

If you start using other things like your 4k High Def TV, laptops, AC cooling unit(s), take a hot shower with electric water heater, cook on electric stove or oven ... yeah, it is really easy to consume 27 kWh from two Powerwall 2 units.The AC units and electric water heater are the heavy consumers in my house. Forget about using the electric house heating at night if you are off grid. Electric house heating is only possible while on grid energy.

For the OP of this topic, one Powerwall is enough to get you through the night during an outage, if you can easily limit your appliances to the bare minimum. A 3.8 kWh solar system should be sufficient to recharge one Powerwall (13.5 kWh) during the next day, if there are a minimum of clouds and the house energy consumption is low during the day.

The primary purpose of the Powerwall, in my opinion, is so that you can go off grid during an emergency. For Florida, that might be after a hurricane. But there are more situations than that.

The week after we got our solar and powerwalls installed, we had a 8 hour grid outage because of some utility damage a few blocks away. My house, and another house down the street with a gas generator, were the only houses with lights. We ran an extension cord to my neighbor so he could have a power strip in his house for fans and cell phone recharging that night.

After seeing my Powerwalls in action, neighbors on each side of me are now planning to get Powerwalls installed and probably solar also. Two referrals I think I have already.

gregbrew | 22 December 2019

I also had an outage (with solar and batteries that backed me up) a few years ago, and several neighbors "were going to get solar and battery backup, too". Not a single one did, and were in the same boat the next time power went out for an extended period.

I only noticed that power was out because neighbors were milling around outside. This was before backup systems would send a text when power went out.

Jones | 24 December 2019

Have had 10kW solar for over a decade. Live in NorCal and have had outages greater than 24 hours at least twice per year (fires, earthquakes, idiots with backhoes, transformer failures - long list). Solar is worthless in those instances - obviously. Added two powerwalls in early spring this year. Have gone into backup mode 15 times for a total of 35 hours since then (longest 31 hours in October due to wildfire shutoff). Worked flawlessly every time. For the October event, I had told my neighbor with a medical condition that I would help him if power ever went out. Power went out at 5am while I was watching the morning news with my wife and I had no idea it was out. Looked at my app at about 630 and saw we were on backup. Got dressed to talk to neighbor and discovered an extension cord plugged into my yard outlet leading over the courtyard wall to his house. Almost fell down laughing. Went over later to say hi. He had run the cord to the center of his house and connected a plug strip and then had extension cords to his fridge and to several bedrooms. At least he had sense to use the plug strip - he was connected to a 15a GFCI circuit so little risk to me.
Love my powerwalls - so does he.

gregbrew | 25 December 2019

Last time my grid power went out (August, transformer failed for 13Hrs), it was ~9PM. I walked my suburban So. CA coastal neighborhood, to find many others doing the same with their flashlights. I told everybody I saw that if they had a medical need for refrigeration or power, to come to my house. They all knew which house mine was, because it was the only one with lights on, and they could see it from several blocks away.

I'm careful to turn off all exterior lighting and reduce interior light intensity during grid failures now. In addition to reducing my visibility, it will also make my PWs last longer. My local neighbors who know me are welcome to visit (and know this), but you never know what whack jobs might come by to try to take advantage of my preparation. This is especially important during long-duration outages (24Hrs+), when people might become desperate, and the rule of law could go out the window. I don't know who said it, but I think it rings true:

"A society is only three meals away from anarchy"

I hate to say it, but if you have Powerwalls, you might consider owning firearms as well.

Patrick | 25 December 2019

Great stories - thx for sharing.

I’ve already offered power to my nearby neighbors in the event of a grid outage. Haven’t had one yet but I’m sure it won’t take them long to roll out the extension cords- it’s only a matter of time.

Probably should have put a few extra outdoor outlets on each side of the home... :-)

PapaSmurf | 26 December 2019

As long as it is reasonably sunny for a few hours each day, we could go indefinitely with two Powerwall 2 units, just with a little logical energy management steps each night.

Then also time the electric hot water heater consumption for the middle of the day when the solar is producing way more than the batteries can store, for example, wash dishes and clothing between 10 am to 2 pm.

With 4 PW2 units, I doubt we would even need to manage things. The entire system would be robust enough to get us thru just about anything except 3 cloudy days in a row. At least one partly sunny day, every third day, would be enough to recover for more cloudy weather.

infofiles | 17 January 2020

Gave up idea on Tesla subscription or purchase. went with LG 9KW system with their most efficient latest technology and got price that was $2.75/watt. I figured using PV Watts calculator, I can easily generate close to 16,000 KWH, my total consumption with Model 3 EV would be about 15,000. I hope I break even in the end. PW would be nice but there will be no ROI on it and batteries will degrade fast into year 10.

my 2 cents, go with larger PV and no PW for now, always add PW later unless you are in fire zone and too many power cuts. I understand that my usage from 4-9 would be 3X the price of other times on TOU Prime (13 cents all day, 4-9 PM its 38 cents). So I will need to not use appliances (washer, dryer, dishwasher, or charge car) between 4-9 PM. rest I can be net exporting the energy.

Google calculator projects 2002 sun hours per year, without system losses project production on 9KW system could be 18K, allow 10% losses, I would easily get 16,000 KWH/year.

Patrick | 18 January 2020

Thanks for sharing the google sunroof portal - had not seen it.

FWIW - as a test I plugged in our FL address but for whatever reason the sizing data was way off. It recommended a 26 kW generator to get a net-zero electric bill, but we have already done this with a 14 kW generator.

As always YMMV....

Patrick | 18 January 2020

Adding some data points after watching our new 10kW microgrid with 3 Powerwalls in action for about 10 days in the NC vacation home.

During that time we only had two mostly sunny days with 38-40 kWh of daily winter generation. The rest of the days were cloudy and rainy with only about 10 kWh of generation. Despite all the bad weather the microgrid self-powered the home for 2-3 days before the PWs ran down to our 10% reserve, at which time we pulled a little juice from the grid until the next morning. Not bad...

During the sunnier winter days we were fully self-powered and the PWs were fully charged by mid-afternoon. Very nice!

So far so good. It looks like being fully self-powered will be no problem during the spring, summer and fall months. Anxious to compare the microgrid performance with our two HVAC systems running in AC mode vs heat mode, however, as I’m guessing they will be more thirsty in AC mode.

We’ll see what happens.